It is well known in the art to utilize gravity downflow filtration tanks divided into a plurality of filter cells, one adjacent the other, and all containing a multi-layer or single layer arrangement of granular filter media such as sand, gravel and the like. In typical downflow filtration systems of this type, water or other liquid containing suspended solid particles is introduced into the filtration tank from above, and clarified water is drawn off from a chamber either beneath or adjacent the individual filter cells. During downward flow through the individual cells, particulate matter is entrapped within the layer or layers of granular filter media, but eventually, the particulate matter clogs the filter media, thereby reducing the filtering capability of the system. Thus, there is a need for a periodic cleaning of the filter cells. It is also known to maintain such units in continuous operation during cleaning by the use of travelling bridge devices which move from one filter cell to the next, cleaning individual cells while permitting the filtration process to continue in the remaining cells.
Examples of travelling bridge apparatus of this type may be found in U.S. Pat. Nos. 5,089,117; 4,988,439; 4,957,631; 4,859,330; 4,764,288; 4,617,131; 4,540,487; 4,486,307; 4,133,766; 3,984,326; 2,235,227; and 2,302,449. Typically travelling bridge systems include an overhead carriage, movable along tracks, guideways or the like, which carry a backwash hood which is located over the open upper end of each filter cell, in either a continuous or step-wise manner. For a downflow type filter, water or other treatment liquid is caused to enter into the cell being backwashed (by a backwash pump) from below in a counterflow arrangement to the normal filtering direction. The backwash hood typically also includes a suction head (i.e., an additional pump) for drawing out fluid and debris forced to the surface of the filter cell as a result of the backwash. As the backwash of individual cells is completed, the travelling bridge moves the backwash hood over the next adjacent cell.
Combined air/water backwash systems are generally used in conjunction with single media, unstratified filter beds, and are usually employed to increase the effectiveness of water only backwash systems. These systems utilize a backwash pump in combination with an auxiliary air supply such as a blower or compressor. Examples of a combined air/water backwash in a travelling bridge filter may be found in U.S. Pat. Nos. 5,078,873; 5,032,294; 4,624,783 and commonly owned U.S. Pat. No. 4,859,330.
This invention relates to an improved air/water (or other liquid) backwash system which incorporates a self-aspirating venturi which expels entrained air or gas through the backwash water discharge pipe, and which utilizes the suction ability of the backwash pump to pull in air for air scour without the use of an auxiliary air supply such as a blower or air compressor. In addition, an automatic actuator on the air supply inlet line allows the filter bed to be pulsed for additional agitation.
In the exemplary embodiment, the overall configuration of the tank, filter bed, individual cell construction and travelling bridge carriage may be in accordance with conventional practice, with exceptions noted. The tank itself is of concrete or steel construction and of generally rectangular shape, defined by a bottom wall, a pair of side walls and a pair of end walls. In an exemplary embodiment, the tank is divided into a plurality of filter cells by a plurality of vertical, transversely extending partitions which extend from one side of the tank to the other in a substantially parallel arrangement. These partitions do not extend to the bottom wall of the tank but, rather, to a horizontal, porous subfloor or underdrain vertically spaced from the bottom wall. Thus, the partitions themselves are fixed to and supported by the side walls of the tank, and, in turn, the partitions support the underdrain. The filter bed media is supported directly on the underdrain so that filtrate from each filer cell drains to a common clearwell below the underdrain and extending substantially the full length and width of the tank. The filtrate is discharged through an effluent outlet at one end of the tank. In alternative arrangements, the filtrate drains through discharge openings in the tank side wall (one opening in each cell) and into a clearwell running the length of the tank.
A trough (divided into separate influent and backwash troughs) extends along an interior surface of one side wall of the tank, adjacent the upper, open end thereof. Influent is uniformly distributed to all of the cells from the influent trough, while backwash or other treatment liquid is carried out of the tank via the backwash trough.
The travelling bridge assembly includes a carriage movable along tracks secured to upper edges of the tank side walls. The carriage supports a backwash hood and associated backwash pump which, in the exemplary embodiment, are suspended from the carriage. The hood itself has length and width dimensions substantially corresponding to individual cell dimensions. The hood may also reciprocate vertically, thereby enabling the hood to sealingly engage each cell, in succession, as the carriage is indexed along the length of the tank.
The backwash hood encloses a backwash header connected to the backwash pump to thereby draw filtrate back up through the cell and into the hood and ultimately to a backwash discharge pipe. The hood also encloses an air inlet pipe for introducing air into the bottom of the cell, as well as air collection chambers for collecting air after it has risen through the cell and entered the hood. A series of air/media separation baffles mounted in the hood serve to separate the air from the backwash water stream, and to separate the filter media which may be carried up into the backwash hood by the violent action of the air scour, from the backwash water stream. As a result, air binding of the pump is prevented and filter media is precluded from being pumped out of the system.
The air collection chambers join with an air discharge pipe which terminates at a backwash liquid venturi in the backwash water discharge pipe, so that the collected air is entrained and discharged with the backwash liquid. The suction created at the venturi is sufficient by itself to pull air into the air inlet pipe (as controlled by the manual and/or automatic damper) without the aid of a compressor blower, as described in greater detail below.
More specifically, air is diverted to air collection chambers on the sides of the backwash hood. Filter media which is carried up into the air/media separation zone settles on the upper surface of the inclined baffles and then falls back into the filter cell area by gravity due to the slope of the inclined baffles. Due to specific gravity and hydraulic design of the backwash hood, only suspended particulate matter which has been removed from the filter bed during backwash and air scour makes its way into the backwash header where it is subsequently expelled from the system by the backwash pump.
To remove the air from the air collection chambers, the chambers are piped to the low pressure port of the venturi type differential pressure ejector located in the backwash pump discharge pipe. This ejector operates under the Bernoulli Principal. Air is drawn into the ejector and is subsequently expelled from the system via an air vent in the backwash water stream.
Each filter cell in this exemplary embodiment is equipped with an air scour manifold which serves to distribute air over the entire area of the filter cell near the bottom of the cell but slightly above the underdrain. The manifold has a central riser pipe which is equipped with a flanged, air scour check valve. A mating gasketed flange is located on the air inlet pipe inside the hood so that when the backwash hood lowers to seal to a pair of cell partitions defining an individual cell, the gasketed flange mates and seals to the air scour check valve. The air supply or inlet pipe inside the hood extends upwardly out of the hood, above the maximum water level in the tank. The air inlet pipe is provided with a manual damper for air volume control, and an automatic damper for cyclic control during backwash.
With the automatic air damper, the system may be operated as 1) a hydraulic only backwash; 2) a pulsed air scour, by cycling the damper to open and closer periodically during backwash; or 3) as a continuous air scour, simultaneous with the backwash. When the system is operated with air scour, the damper is programmed to close prior to de-energizing the backwash pump so that entrained air can be expelled before returning the system to the dosing mode. Entrained air, if not expelled from the filter media, will air bind the filter bed, inhibiting the dosing mode.
The above generally described travelling bridge filtration system operates in a dosing, i.e., filtration mode and in a backwash mode. The dosing or filtration mode is substantially the same as any conventional gravity (i.e., downflow) filtration device. The backwash mode is initiated by head loss or by timer and, when so initiated, the bridge travels to the first cell and stops. The backwash hood lowers and seals to the cell. After a short pause, the backwash pump energizes and begins to draw filtrate up through the underdrain, through the filter media, into the backwash hood where it is collected by the backwash header which is coupled to the backwash pump. Backwash flow is discharged by the backwash pump past a venturi then through a back pressure rate control valve and then into the backwash waste trough.
The underdrain system is designed to induce sufficient hydraulic headloss at the design backwash rate to insure uniform hydraulic flow. At this design headloss, the filter cell operates under negative pressure during backwash. As backwash liquid flows past the venturi located on the backwash discharge pipe, air is drawn down through the air scour riser pipe, into the air scour manifold distribution system and into the filter cell where it rises through the filter media concurrently with the backwash water thereby providing a very violent agitation of the filter media. The backwash air is entrained with the backwash liquid at the venturi (downstream of the backwash pump) and both air and backwash liquid are discharged together.
The backwash pump discharge line is equipped with a rate control valve to set operating conditions. This valve is located downstream of the venturi. As noted above, an air vent is located in the discharge line downstream of the rate control valve to release the entrained air, which would otherwise cause surging at the backwash water discharge point.
Thus, in accordance with the broader aspects of one exemplary embodiment of the invention, there is provided a travelling bridge filtration system including at least one filter cell adapted to be periodically backwashed, a combined air/water backwash apparatus comprising backwash liquid pump means for effecting the introduction and removal of backwash air and liquid into and out of the cell in a direction counter a normal filtering direction.
The invention also relates to a method of backwashing a filter cell with both air and liquid broadly comprising the steps of:
a. locating a backwash hood over the filter cell; PA1 b. activating a backwash liquid pump to draw backwash liquid up through the filter cell in a direction counter to a normal filtration flow direction and into the backwash hood; and PA1 c. simultaneously with step (b), drawing air up through the filter cell with the backwash liquid, using suction created by the backwash liquid pump. PA1 a. waste water denitrification; PA1 b. secondary waste water filtration; PA1 c. potable water filtration; PA1 d. industrial process water filtration; and PA1 e. industrial waste water filtration.
Other more specific aspects of the process steps used to carry out the combined liquid/air backwash will become apparent from the detailed description further herein.
The above described combined air-water backwash system has applicability to: